1. Field of the Invention
The present invention relates generally to memory devices and, more particularly, to a non-volatile magnetic memory devices.
2. Description of Related Art
Integrated circuits, typically in the form of microprocessors, microcontrollers, or other logic circuits, are used to control the functions of many modern electronic devices. For example, integrated circuits are used to control the functions of computers, telephones, and many other consumer electronics. It is generally necessary for the integrated circuits to retrieve (read) and store (write) data as they perform their functions. The data may be in the form of instructions for the integrated circuits (e.g., a program), data necessary for the execution of a program, or data generated during the execution of the program. It is preferable to store the data in memory devices which are easily accessible by the integrated circuits.
Many different types of memory devices are known for the storage of data. In selecting a memory device, the particular requirements for the data with which the memory device will be used are important. For example, several parameters such as the quantity of data, the required access time and the required storage time can play an influential role in memory device selection.
Magnetic random access memory (MRAM) is a type of memory that retains stored information after power has been removed from the memory device. This type of memory is called non-volatile memory. Magnetic random access memory has lower power consumption than short-term memory such as dynamic random access memory (DRAM), static random access memory (SRAM) or Flash memory. Magnetic random access memory can perform read and write operations much faster (by orders of magnitude) than conventional long-term storage devices such as hard drives. In addition, magnetic memory is more compact and consumes less power than hard drives.
A typical magnetic memory device includes an array of memory cells with word lines extending along rows of the memory cells, and bit lines extending along columns of the memory cells. Memory cells are located at intersections of the word lines and bit lines. A typical magnetic memory cell includes a magnetic pinned layer, a magnetic free layer, an insulating barrier sandwiched between the pinned and free layers and a semiconductor diode located between the memory cell and a substrate. The pinned layer has a magnetization orientation that is fixed so as not to rotate in the presence of an applied magnetic field in a range of interest. The free layer has a magnetization that can be oriented in either of two directions: the same direction as the pinned layer magnetization or the opposite direction of the pinned layer magnetization. These two magnetization directions can correspond to logic values of 0 and 1 in a memory system. The semiconductor diode prevents leakage currents from flowing between adjacent memory cells in the array. Such leakage currents can lead to inaccurate or erroneous readings of the contents of the memory cell.
Methods of making and designs for magnetic memories are disclosed in U.S. Pat. No. 5,838,608 to Zhu et al; U.S. Pat. No. 5,793,697 to Scheuerlein; U.S. Pat. No. 5,930,164 to Zhu; U.S. Pat. No. 5,801,984 to Parkin and U.S. Pat. No. 6,347,049 to Childress et al, all of which are hereby incorporated by reference in their entireties. As set fourth in the prior art, a typical magnetic tunnel junction memory cell requires a semiconductor diode in series with the cell in order to prevent leakage currents from flowing to adjacent cells. Unfortunately, the semiconductor diode can be difficult to fabricate on the memory cell, can add additional expense and can lead to lower yields. Another problem exists with the storage times of magnetic memory cells. When the contents of a magnetic memory cell are read by the application of an electric current, some of magnetic domains can be slightly mis-oriented from their original orientation. After a number of read cycles, the magnetic orientation can be degraded from what was originally written to the cell, thus causing a shorter retention or storage time for data contained within the memory cell.
A need thus exists in the prior art for a magnetic memory that does not require a separate diode to prevent leakage currents for the device. A further need exists for a magnetic memory cell that has an improved storage life.